I don't know how a question of what you define as a sidequest can be turned back to me...are you asking what I consider a sidequest?
An optional task that requires combat, puzzle solving, winning a "mini game", or any level of skill to earn a finite game resource or unlocks a change in the game such as a new power, new outfit, different looking overworld, etc.
Almost anything that ends with a heart piece would be considered a sidequest. Almost all things that end with earning a bottle would be called a sidequest, but some bottles (like the first one usually) are given the main quest while others (2nd bottle in SS) are just given out for free. Grinding for treasure or rupees is NOT a sidequest. Playing a mini game is only a sidequest if the reward given is finite. Cave of Ordeals, Savage Labyrinth, and Lightning Round are all sidequests. The golden skulltulas are a sidequest, even though the end reward is only rupees because collecting all 100 creates a change in the game (spiders turn to humans). And lastly, buying something from a shop with currency isn't a sidequest, no matter what it is.